Electrostatic printing



s. w. JoHNsoN ETAL 3,121,010

ELECTROSTATIC PRINTING Filed March 22, 1961 Feb. 1 1, 1964 nm; f /la/caW//mff/wf/m' mwa/,va Aff/mie.

Pia/far m /fr'aif /r /fJff/ii ra may mia w Minne-faenza INVENTOR5 United States Patent O 3,121,010 ELECTRSTATIC PRINTING Sigurd W. Johnson, Oaklyn, and John P. Lauriello, Westmont, NJ., assignors to Radio Corporation of America, a corporation of Delaware Filed Mar. 22, 1961, Ser. No. 97,989 3 Claims. (Cl. ilo- 1) The present invention relates to improved methods of electrostatic printing and more particularly to improved methods of electrophotography.

This is a continuation-in-part of our copending application Serial Number 797,186, led March 4, 1959, now abandoned.

Commonly used electrophotographic media are of a very high contrast nature. These normally have a surface formed of a photoconductive insulating substance. The continuous tone reproduction of the medium can be greatly improved by resolving the image into dots as in some photoengraving processes, for example. A screen is commonly employed for this purpose. In present electrophotographic methods using screens, the screen is commonly placed in contact with the picture material and light is projected through the screen and the picture onto a charged electrophotographic medium. The quality of the picture thus produced is high and the continuous tonal range is good. However, the exposure time is relatively long-4, 5 or more times that required for reproducing the same picture without the screen.

The object of the present invention is to provide a more rapid method for producing a screened copy by an electrophotographic process.

According to the invention, a high contrast electro photographic medium is charged and an electrostatic image of the screen formed thereon. The time required to form the image can 'oe very short. The medium is then recharged and a continuous tone image of the picture desired to be reproduced is exposed thereon. The exposure time is the normal exposure time for reproducing the picture. The electrostatic image is then developed and fixed in the normal manner. The resultant copy is equal to quality to one obtained using the conventional screening method previously described but the time required is only a fraction of that required with the conventional method.

The invention will be described in greater detail by reference to the following description taken in connection with the accompanying drawing, the single figure of which is a block diagram of the process of the present invention.

In practicing the process shown in the figure, the photoconductive insulating surface of a high contrast electrophotographic printing medium is charged in the normal fashion. Such charging is conveniently accomplished by subjecting the surface to corona discharge as, for example, by passing thereover an array of ne wires to which has been applied a corona generating potential of the order of i4,500 to 10,000 volts. There is no criticality with respect to the potential applied to the wires other than that it be sufficient to produce corona along the length of the wires. ln practice, the corona generating potential is very much higher than the top voltage to which a photoconductive surface can be charged so that variations in corona potential will not signicantly effect the charging operation. White light is then projected through a screen onto the charged paper. The screen may be any one of a number of screens used in the engraving or other more conventional graphic arts. Typical screens are described in the Ilford Manual of Process Work, by L. P. Clerc, 5th edition, Chapter 13. After the electrostatic screen image is formed on the photoconductive surface, it is recharged over-all in substantially the same manner and in the 3,121,0l0 Patented Feb. l1, 1964 same polarity as before. A continuous tone original is then exposed to the surface by projecting light through the original and onto the charged surface. The latent image is then developed in the usual fashion by applying an electroscopic toner and then xing by fusing or some equivalent method. The toning may be done in any known fashion such as by a dry or wet technique. Various procedures of this type are described in an article by Young and Greig appearing in the December 1954 issue of RCA Review on page 469 and in an article by Metcalfe and Wright appearing in the November 1956 issue of the Journal of Oil and Colour Chemist Association (British) at page 845.

It has been found, in the practical operation of the process described, that the pre-exposure of the electrophotographic medium to the screen image produces a latent screen image on its photoconductive surface. The recharging of the surface does not erase the screen image. The reason, it is believed, is that, for an appreciable length of time, the photoconductive insulating surfaces normally employed in electrophotography have a built-in memory system. This memory system is sometimes called a fatigue eifect. Once such a surface area has been exposed to light it remembers the exposure and, upon recharging with the same polarity of charge, an exposed area cannot be brought to as high a surface potential as an area that has not been previously exposed. Thus, areas under the dots (the areas not discharged by the light passing through the screen) can readily be recharged in the same polarity to their original top voltage which may be 400 or more Volts, but the areas in between the dots (the areas discharged by light passing through the screen) can only be partially recharged i.e. the surface voltage in the latter areas may be as much as volts less than top voltage. In a subsequent exposure to a continuous tone image, light of relatively low intensity will be sufficient to discharge areas between dots while the dot areas will require high intensity light to produce discharge thereof. Light of intermediate intensity (shades of grey) will provide an eect equivalent to varying the size of the charged dot areas in accordance ith a grey scale. The subsequent exposure to the continuous `tone original thus produces on the photoconductive surface a latent image of the original over the latent image of the screen just as if the screen and the original were exposed together onto the surface. lt has also been found that the amount of contrast in the linal print can be controlled by varying the pre-exposure time of the screen to the surface. The greater the length of time light is allowed to pass through the screen or the greater the intensity of the light passing through the screen, the more contrasty the final print. This result follows since the greater the pre-exposure time, the smaller the latent image of each of the dots on the paper and the larger the latent image of each of the exposed areas on the paper. A long exposure through the screen will have the effect of erasing the dots resulting in a more contrasty iinal print.

There are numerous advantages to the method of screening of the present invention. With the screen exposed to the paper in a few seconds or so before the original picture is exposed, the total exposure time required for the original is only the normal second or two. It the screen and picture are exposed to the electrostatic printing paper together, as is presently done, the exposure time is very much longer.

The method of the present invention is especially adaptable to automatic electrostatic printing such as is now used for oiiice copiers, library copiers, or the like. The pre-exposure to the screen in this case is done automatically immediately after the original charging step by having the original charge exposed to a drum type screen.

Lens Screen Pre- Picture Paper Opening exposure, Exposure,

see. sec.

A f2.2 2-4 2 B f22 2-4 3 Paper A was coated With the following:

(1) 640 parts silicone Sil-82 solution (60% solids in xylene) (2) 840 parts toluene (3) 960 parts of White zinc oxide-French Process Florence Green Seal pigment grade, marketed by the New Jersey Zinc Sales Company, Inc., New York, N Y.

(4) .14 part uorescein sodium (5) .048 part rose bengal (6) .18 part brilliant green Upon evaporation of the solvents a dye-sensitized photoconductive insulating layer is formed on and bonded to the paper.

Paper B-iterns (1) (2) and (3) above in quantities noted (4) .100 part patent blue (5) .040 part rose bengal.

If the screened exposure were made according to the prior art method, the time required for paper A would be about 8-15 seconds or more and for paper B about 12-18 seconds or more.

What is claimed is:

1. A method of electrophotography comprising the steps of substantially uniformly exposing a photoconductive insulating surface to corona discharge of one p0- larit exposing said surface to a screen pattern light image; substantially uniformly exposing said surfaceVv to corona discharge of said one polarity; exposing said surface to a continuous tone light image; and applying to said surface an electroscopic toner to produce thereon a visible screened reproduction of said original light image.

2. A method of electrophotography comprising the steps of substantially uniformly exposing a photoconductive insulating surface to corona discharge of one polarity; exposing said surface to light passing through a photographic screen while said screen is in contact with said surface; exposing said surface to corona discharge of said one polarity; exposing said surface to-a continuous tone light image; and applying to said surface an electro-` scopic toner to produce thereon a visible screened reproduction of said original light image.

3. A method of electrophotography comprising the steps of: substantially uniformly exposing a photoconductive insulating surface to corona discharge of one polarity; juxtaposing on said surface a photographic screen; exposing said surface to light passing through said screen; again substantially uniformly exposing said surface to said corona discharge; juxtaposing a continuous tone transparency on said surface; exposing said surface to light passing through said transparency; and, applying an electroscopic toner to said surface to produce thereon a screened reproduction of said continuous tone transparency.

Walkup a June 3, 1952 Giaimo Apr. 1l, 1961 

1. A METHOD OF ELECTROPHOTOGRAPHY COMPRISING THE STEPS OF SUBSTANTIALLY UNIFORMLY EXPOSING A PHOTOCONDUCTIVE INSULATING SURFACE TO CORONA DISCHARGE OF ONE POLARITY; EXPOSING SAID SURFACE TO A SCREEN PATTERN LIGHT IMAGE; SUBSTANTIALLY UNIFORMLY EXPOSING SAID SURFACE TO CORONA DISCHARGE OF SAID ONE POLARITY; EXPOSING SAID SURFACE TO A CONTINUOUS TONE LIGHT IMAGE; AND APPLYING TO SAID SURFACE AN ELECTROSCOPIC TONER TO PRODUCE THERON A VISIBLE SCREENED REPRODUCTION OF SAID ORIGINAL LIGHT IMAGE. 